1. Field of the Invention
The present invention relates to a calibration apparatus and a calibration method configured to calibrate a spectrometer according to a correspondence relation between a pixel position and a wavelength corresponding to a bright line spectrum of a sensor.
2. Description of the Related Art
FIG. 10 illustrates an example of a Rowland spectrometer including a combination of a concave diffraction grating and a light-receiving sensor (a line sensor). Referring to FIG. 10, a sample is irradiated with light from a light source. Reflection light from the sample is incident to the spectrometer via a slit. Then, the light incident to the concave diffraction grating is dispersed and reflected. Further, the reflected light is formed on the concave diffraction grating as an image on a circle whose diameter is equivalent to a radius of curvature. Spectral information about the incident light can be measured according to an intensity of the spectral light acquired as a pixel value by a line sensor, which is provided at an image forming position. If a spectrometer having the above-described configuration is used, the spectral information cannot be measured at a high accuracy unless the correspondence relation between the wavelength of the incident light and a pixel position on the line sensor is accurately calculated.
In the spectrometer having the above-described configuration, the correspondence relation between the pixel position and the wavelength becomes nonlinear due to the following two characteristics. More specifically, firstly, an image of optically dispersed light is formed along a circumference but the line sensor generally has a flat surface. Accordingly, the position at which an image has been intended to be formed and the position on the line sensor may become different from each other. Secondly, the dispersed light may be defocused due to the optical characteristic of the concave diffraction grating. In addition, the defocus characteristic may differ according to the wavelength.
In order to calculate the nonlinear correspondence relation between the pixel position and the wavelength, a method is generally used in which a single wavelength is caused to enter the spectrometer and the correspondence relation is calculated for each pixel using a monochromator. However, it is not practical to assemble a spectrometer having a monochromator into an apparatus such as a printer.
Japanese Registered Patent No. 2689707 discusses the following simple calibration method. More specifically, in this conventional method, light emitted from a light source having a plurality of bright line spectra is caused to enter a spectrometer. Further, the wavelength of the bright line spectrum and a corresponding detection position is calculated. Moreover, a regression curve is generated based on a combination of the wavelengths of the plurality of bright line spectra and the corresponding detection positions. Thus, the conventional method calculates the correspondence relation between the pixel position and the wavelength. Hereinbelow, a “detection position” refers to a pixel position on a sensor that corresponds to a specific bright line spectrum.
If the method discussed in the Japanese Registered Patent No. 2689707 is used, in order to calculate the correspondence relation between the pixel position and the wavelength, it becomes necessary to previously specify the range of the detection positions at which an image of a bright line spectrum is formed. However, in a spectrometer, the wavelength and the detection position on the sensor may vary due to aging of the spectrometer or changes in the temperature inside the spectrometer. Accordingly, the correspondence relation between the detection position and the wavelength may not always be appropriately acquired within a previously specified range. As a result, the spectrometer may not be accurately calibrated.